1. Field of the Invention
The invention relates to a sidelobe-canceller system for cancelling jamming interference signals from a radar signal. More specifically, the sidelobe-canceller system utilizes preprocessing improved by the addition of an AGC circuit to stabilize gain and therefore improve the cancellation ratio and transient response of the system.
2. Description of the Prior Art
Generally, signal-processing systems are designed to reduce the presence of undesired signals in a signal-receiving system. As is known, in particular systems, such as a radar system, the characteristics of the receiving antenna are such that undesired signals which are received in the antenna sidelobes interfere with the detection of a target signal received in the main lobe. Accordingly, to reduce interference entering the sidelobes, sidelobe-canceller systems using adaptive canceller loops (hereinafter cancellers) have been used to cancel interference from the sidelobes of the main-radar antenna as exemplified by U.S. Pat. No. 3,202,990 to Paul W. Howells.
Conventionally, receiving systems employing cancellers have used auxiliary receivers to sample the environment in which undesired interference signals may be present. Each auxiliary signal forms an input which is used by a canceller in an attempt to eliminate undesired signals which may be present in conjunction with the desired signal in a main channel. Each canceller attempts to decorrelate its output from an auxiliary input signal by first phase shifting and amplitude weighting the auxiliary input signal and then subtracting this modified auxiliary signal from the main-channel signal.
Such systems as described above work well to eliminate main-channel interference with a single canceller when only one interference source is present. However, when multiple interference sources are involved, conventional systems have been ineffective in reducing cancellation even though multiple cancellers have been used. In theory, if N independent interference sources are present in a signal environment, the interference signals may be cancelled by multiple cancellers fed by inputs from N separate auxiliary receivers. In practice, however, it has been found that effective cancellation cannot be obtained unless the auxiliary-signal inputs are independent (i.e. decorrelated) of one another, in order to prevent the reintroduction of signals which have been cancelled in a previous main-channel canceller. In addition, conventional systems using parallel-connected cancellers have exhibited instabilities at the gains required for effective cancellation, and insufficient cancellation when constructed and operated over practically obtainable dynamic ranges.
In U.S. Pat. Nos. 3,938,153 and 3,938,154 entitled "SIDELOBE CANCELLER SYSTEM" to Bernard L. Lewis and Irwin B. Olin and "MODIFIED SIDELOBE CANCELLER SYSTEM" to Bernard L. Lewis, and assigned to the same assignee as the present application, iterative systems are disclosed which increase the cancellation over that of conventional systems. These techniques, however, require many iterative loops, with each loop requiring multiple cancellers before complete cancellation can be obtained.
In copending Patent Applications, Ser. No. 499,357 entitled "ADAPTIVE SIDELOBE CANCELLER SYSTEM" by Bernard L. Lewis and Ser. No. 508,774 entitled "SAMPLED DATA PROCESSING" by James P. Hansen, both assigned to the same assignee as the present application, systems are disclosed which preprocess the auxiliary signals to provide a plurality of independent signals which are used in the main-channel canceller system to cancel interference. Patent Application Ser. No. 508,774 describes a system using multiple preprocessing cancellers with each preprocessing canceller operating on the preprocessed output of the previous preprocessing canceller. This allows independent samples to be formed with a minimum number of preprocessing cancellers. These systems still have the disadvantage of having uncontrolled gain especially in the last main-channel canceller. This results in less than an optimum cancellation ratio and transient response.
Additionally, cancellers have used AGC circuits internally to stabilize gain. However, these circuits have been unable to completely control gain of the canceller since the gain is still variable with the auxiliary input signal.